Acidosis in CKD May Affect Mineralization of Newly Formed Bone According to HR-pQCT and Quantitative Back Scatter Electron Imaging
Authors: Matthew J. Williams, Keith A. Hruska
Journal: Journal of the American Society of Nephrology (2023)

CKD is an increasingly prevalent condition with a high mortality risk. Its management often produces a focus on its complications, one of which is the CKD-mineral bone disorder (CKD-MBD). The CKD-MBD is an important contributor to the cardiovascular mortality risk associated with CKD.1 The CKD-MBD is a complex multifactorial syndrome consisting of skeletal, vascular, and cardiac organ involvement; abnormalities in four hormones—parathyroid hormone (PTH), calcitriol, fibroblast growth factor 23 (FGF23), and circulating α-klotho (klotho); and disordered calcium and phosphorus mineral metabolism.2 The skeletal disorder, renal osteodystrophy (ROD), is a classical component woven into the CKD-MBD syndrome and often referred to by one of its key pathogenic mechanisms, secondary hyperparathyroidism. ROD is a disorder of bone turnover and mineralization producing diminished bone quality and strength leading to increased fracture risk and mortality and contributing to the disordered phosphorus and calcium metabolism. The pathogenesis of ROD is complex, and progress since the incarnation of the CKD-MBD syndrome has shown that factors produced by diseased kidneys cause systemic complications, including ROD. Included in the factors produced by diseased kidneys are inhibitors of Wnt (portmanteau of Wingless and Int1) signaling that circulate and decrease skeletal metabolism, which in part, is under the control of Wnts.3 Early in CKD (stage 2–3), the Wnt inhibitors produce a decrease in Wnt-regulated bone turnover, which as CKD progresses is converted to a higher turnover and lower quality state by PTH and other factors.4,5 Among the other factors, contributing to ROD as CKD progresses is metabolic acidosis. The onset of acidosis in CKD is usually stage 3b or 4. Acidosis in CKD is due to decreased ammonia excretion and decreased bicarbonate reabsorption in the proximal tubule. Acidosis is limited because retained protons are buffered in the skeleton.6 Acidosis inhibits skeletal mineralization because calcium phosphate crystallization and hydroxy apatite formation are pH sensitive. However, the causative role of acidosis in ROD is unknown. Older studies in the era of aluminum-based phosphate binders demonstrated that correction of acidosis decreased osteomalacia, but these data are out of date because of elimination of aluminum. An up to date characterization of the effects of acidosis on the skeleton in CKD is absent. Levy et al. in this issue of JASN attempt to correct this gap in our knowledge.7 In a secondary analysis of a study conducted at Columbia University by Nickolas and collaborators from 2006–2011 to characterize ROD, Levy et al. divide the cohort studied into those with and without acidosis. The cohort consisted of 180 subjects available for cross-sectional analysis, 56 subjects studied longitudinally for a median of 1.5 years, and 22 subjects who underwent bone biopsies (ten acidotic). Seventy-two of the 180 subjects were acidotic, and 20 of the 56 studied longitudinally were acidotic. Besides the standard serum/plasma bone biomarkers, subjects underwent dual-energy x-ray absorptiometry (DXA) and high-resolution peripheral computed tomography (HR-pQCT). Bone biopsies were assessed by histomorphometry, microCT, and quantitative backscatter electron microscopy (qBEI). Associations with acidosis were determined. Besides the multiple methods of study, there were numerous determinations within each method. The main findings divided by cross-sectional and longitudinal analysis and by type of analysis follow below. Cross-Sectionally By Serum/Plasma Biomarker Participants with metabolic acidosis had lower levels of mean eGFR, 25-hydroxy vitamin D, the Wnt signaling antagonist sclerostin, and FGF23. Acidotic subjects had higher levels of phosphorus, intact PTH, markers of bone formation (P1NP, osteocalcin), and resorption (CTX, TRAP5b). By Bone Imaging—DXA (1) Subjects with acidosis had lower areal bone mineral density at the one-third radius. By HR-pQCT (1) In women with acidosis, trabecular density and microarchitecture were impaired and cortices were thinner. Longitudinally By DXA There were no relationships between changes in bone outcomes and either the baseline or change in bicarbonate. By HR-pQCT More severe acidosis was associated with the following: (1) Decreased tissue mineral density and lower and more heterogenous calcium content than nonacidotic participants. (2) More severe acidosis at baseline and worsening acidosis over time were related to decreases in trabecular number and heterogeneity of the trabecular network. Worsening acidosis over time was related to expansion of the cortex (increases in cortical area and perimeter) and increases in cortical density. By Bone Biopsy The authors report relationships between severity of acidosis and bone turnover and mineralization and volume from histomorphometry. They also determined three-dimensional trabecular microarchitecture and tissue mineral density from microCT and the content and distribution of calcium in bone tissue by qBEI. Bayesian estimates demonstrated that more severe acidosis was associated with lower tissue mineral density (ρ=0.60, P=0.004) by microCT. By qBEI, there was a shift of calcium content distributions to lower levels and a broader distribution (i.e., more heterogeneous calcium content) in trabecular and cortical bone. In exploratory analyses, lower and more heterogenous tissue mineral content was related to cortical geometric alterations. The major strength of the study by Levy et al. is its comprehensive analysis combining serum/plasma biochemistry, DXA, HR-pQCT, and bone biopsy. The comprehensive methods of analysis also lead to a weakness—the number of comparisons. There were so many comparisons performed, their number was larger than the sample size, and the study was not fully corrected for multiple testing. This necessitated a descriptive rather than hypothesis testing approach to the data because the probability of false positives was high. There were other important weaknesses in the study, which the authors acknowledge. First, the secondary analysis of the original study was not prespecified. The study is more than a decade old, but CKD treatment and skeletal analytical techniques are still modern. Second, the cohort size for the longitudinal analysis and bone biopsy was small, but they were at least performed. In addition, the associations between acidosis and the bone analyses reported are just that and not causality. In fact, the findings reported are similar to the effects of other factors within the CKD-MBD, such as PTH, and activin receptor signaling, that is, increased osteoclast stimulated turnover and decreased osteoblast function. Indeed, the authors stated that "elevated time-averaged levels of PTH and bone turnover markers predicted cortical deterioration" by HR-pQCT in a previous report of the longitudinal cohort reported here.8 However, the qBEI findings reported here have not been reported for PTH effects and may be specific to acidosis (Figure 1). Another issue is that the HR-pQCT findings were modeled for changes in eGFR so that the acidosis cohort, which had lower eGFR, did not select duration and severity of CKD.Figure 1: Deduced pathogensis of acidosis stimulated ROD. Stage 3b and 4 CKD produce metabolic acidosis. Increased proton levels impair hydroxyl apatite formation on newly formed collagen fibrils in the mineralization fronts of bone formation. This weakens newly formed bone. In addition, proton stimulation of RANKL production in osteoblasts stimulates osteoclastogenesis and NFATc1 stimulation in osteoclasts leading to increased bone turnover. The hypomineralized trabeculae are subject to breaks and resorption decreasing trabecular bone density. The weakened bone structures subjected to weight-bearing result in cortical expansion and fracture risk. NFATc1, nuclear factor of activated T cells.In summary, the results of a comprehensive analysis of ROD in subjects with and without acidosis demonstrate associations that concur with our understanding of skeletal biochemistry, cellular biology, and pathophysiology. This leads to an advance in our understanding of how acidosis contributes to ROD in CKD. As progressive CKD advances to late stage 3b and 4, decreased proton excretion and bicarbonate reabsorption produce acidosis biomarked by reduced serum HCO3 levels (Figure 1). The increased proton levels are limited by skeletal buffering. The increased proton levels impair apatite formation and deposition on newly formed type 1 collagen fibrils deposited by osteoblasts in the mineralization fronts. The hypomineralization of newly formed bone was demonstrated by Levy et al. in their microCT and qBEI analyses showing decreased tissue mineral density and a broader distribution of calcium content in samples from acidotic subjects. In trabecular bone, the hypomineralized trabeculae are more prone to break leading to resorption and decreased trabecular number and heterogeneity. Increased osteoclast cytosolic protons stimulate nuclear factor of activated T cells and osteoclast-mediated resorption and bone turnover.9 Increased bone resorption from the osteoclasts leads to increased bone formation rates as shown in the histomorphometry of Levy et al. The newly formed hypomineralized and weaker bone subjected to weight-bearing leads to cortical bone expansion as shown by HR-pQCT according to Wolf law (Figure 1). The pathophysiology surmised by these studies suggests that intervention and normalizing HCO3 in late CKD should be efficacious. The failure to document this by Melamed et al. was most likely related to experimental design and the primary endpoint, DXA.10 As shown in the study by Levy et al., DXA was an insensitive endpoint for the effects of acidosis.7 As a result, the studies by Levy et al. generate hypothesis development for future analysis. Future study is strongly begged because acidosis correction should be met with little side effect, and the intervention should be efficacious. Disclosures None. Funding Funding was received from the National Institute of Diabetes and Digestive and Kidney Diseases through grant RO1 DK127186. Author Contributions Conceptualization: Keith A. Hruska. Data curation: Matthew J. Williams. Funding acquisition: Keith A. Hruska. Visualization: Matthew J. Williams. Writing – original draft: Keith A. Hruska. Writing – review & editing: Keith A. Hruska, Matthew J. Williams.